1. Field of the Invention
The present invention generally pertains to integrated optical circuits, and particularly to a method for manufacturing an integrated optical circuit.
2. Technical Background
Integrated optical circuits are usually constituted by optical and/or optoelectronic devices formed on a substrate. The optical and/or optoelectronic devices may be active devices (such as resonators, lasers, detectors, switches, etc.) or passive devices (such as waveguides, filters, couplers, etc.)
An interesting feature of integrated optical circuits resides in their compactness. However, these circuits experience optical losses as light propagates from one device to another on the substrate.
Light between the optical devices of an integrated circuit propagates through waveguides. Conventionally, waveguides are composed of a continuous structure made of a material having a higher refractive index than the surrounding substrate. Light propagates inside the waveguide by virtue of internal reflections off the faces thereof. Introduction of light into the waveguide is carried out so that the angle of reflection off the internal faces of the waveguide, as light propagates, is smaller than a critical angle depending on the refractive indices inside and outside the waveguide. This angle requirement may generally be fulfilled when the waveguide is in the form of a straight line. However, when the waveguide is bent (bent portions are needed in integrated circuits in order to reduce their size), optical losses occur due to the fact that the reflection angle at the faces of the waveguide is too large, which gives rise to refractive waves.
One technique for reducing optical losses in integrated optical circuits, using a photonic crystal, consists in providing, around optical devices, a dielectric periodic structure which creates a frequency band gap. A frequency band gap is a range of frequencies (or photonic energy) at which propagation through the periodic structure is impossible. The parameters of the dielectric periodic structure, such as the period length, the refractive index of the structure, the shape of the periodic lattice, etc., are calculated so that the frequency of the light propagating inside the waveguides that interconnect the various optical circuits on the substrate is within the frequency band gap. Thus light remains confined inside the optical circuits and waveguides since it cannot radiate outside these optical devices, in the dielectric periodic structure.
FIG. 1 illustrates an example of such an integrated optical circuit. For the purpose of simplification, the integrated optical circuit of FIG. 1 essentially consists of a waveguide 1 surrounded by a dielectric periodic structure 2. The waveguide 1 receives light on one lateral side thereof as shown by arrow 3. Light propagates inside the waveguide to emerge from the other lateral side as shown by arrow 4.
The integrated circuit comprises a substrate 5 coated with a dielectric layer 6 having a higher refractive index than the substrate 5. The dielectric periodic structure 2 is arranged along the longitudinal sides of the waveguide 1. The structure 2 typically consists of an array of holes 2a formed throughout the layer 6 and substrate 5, perpendicularly to the longitudinal axis of the waveguide 1, but, as a variant, may also consist of an array of rods. The waveguide 1 is defined by the central portion of the dielectric layer 6 bounded by the periodic structure 2. Horizontal radiation of light out of the longitudinal sides of the waveguide 1 is prevented by the periodic structure 2, which reflects, according to a diffraction phenomenon, optical waves having frequencies within the corresponding band gap. Light is further vertically confined within the waveguide by virtue of the fact that the refractive index of the waveguide is higher than that of the substrate 5 and that of air. Alternatively, it is also possible to provide a three-dimensional dielectric periodic structure around the waveguide, instead of the two-dimensional structure as illustrated in FIG. 1, in order to confine the light within the waveguide both horizontally and vertically.
The present invention provides a reliable method for manufacturing an integrated optical circuit in which an optical device, such as a waveguide, a resonator, etc., is associated with an optical structure, such as an array structure defining a frequency band gap region. Both the optical device and the optical structure may be formed with great accuracy.
The method generally comprises the steps of: forming a first mask on a face of a substrate, the first mask defining a pattern corresponding to at least one optical device to be formed in a first region of the substrate; forming a second mask on the face of the substrate, the second mask defining a pattern corresponding to an optical structure to be formed in a second region of the substrate, distinct from the first region; and etching the substrate having thereon the first and second masks, in order to form the at least one optical device and the optical structure in the substrate.
By virtue of the simultaneous use of the first and second masks, each one corresponding to a region of the substrate, it becomes possible to form the optical device(s) and the optical structure(s) on the substrate in a simple manner and with great accuracy.
A reason for this is that the two masks may be formed separately and, therefore, two respective specific patterning methods may be applied to form the two masks. In other words, the patterning technique used to form the second mask, corresponding to the second region of the substrate, may be different from that used to form the first mask, corresponding to the first region of the substrate.
This is particularly important, especially in the context of photonic crystals, when it is desired to manufacture integrated optical circuits having one or more optical devices, such as waveguides, couplers, etc., and an array structure defining a photonic band gap proximate to the optical device(s). Indeed, in such a case, the optical device(s) and the array structure have quite different shapes. The array structure to be formed in the substrate is periodic or quasi periodic, with a period which may be small, in the order of 250-500 nm. The construction of this periodic structure requires a specific technique suitable for forming a mask with small irregularities, such as holes, disposed according to a periodic lattice. A typical such technique may consist of interference or holographic lithography, which uses two interfering laser beams irradiating the wafer. Interference lithography however is not appropriate for forming a mask corresponding to the optical devices (waveguides, resonators, couplers . . . ), because these devices generally do not consist of a periodic or quasi periodic structure. A conventional lithography technique employing UV (ultra-violet) exposure will, on the contrary, enable such a mask to be satisfactorily formed. Thus, the present invention makes it possible to select, for each of the first and second regions of the substrate, a specific appropriate patterning technique for forming the corresponding mask.
By contrast, using a single mask for both the optical device(s) and the optical structure would require, in many cases, a complicated lithography method for producing the mask, namely a method which would be adapted to all kinds of shape contained in the pattern to be created in the substrate. The known UV or electron beam exposure lithography techniques, although suitable for producing masks corresponding to optical devices such as waveguides, lasers, etc., cannot offer, for the time being, a sufficient accuracy for forming frequency band gap structures.
Another advantage of the present invention resides in that the two masks are used simultaneously, so as to form the optical device(s) and the optical structure together. The present invention thus provides a simple process, requiring only one etching step for the substrate.
Preferably, the method according to the invention further comprises the step of removing the first and second masks.
The step of etching the substrate may consist of a dry etching step using a predetermined etching gas, for example a fluorine-bearing gas such as SF6. In this case, the first and second masks are each made of a material which substantially resists the predetermined etching gas.
Preferably, in order to simplify the manufacturing method, the step of forming the first mask and the step of forming the second mask are carried out in such a manner that one of the first and second masks overlays the other. There is then no need to define two separate zones where, respectively, the first and second masks have to be formed, since one of the two masks may overlay the other. In practice, one of the first and second masks has a first portion which overlays the other mask and a second portion which is in direct contact with the substrate.
According to the invention, one of the first and second masks may be formed using an interference lithography technique, whereas the other may be formed using a UV exposure technique.
Advantageously, the second mask is formed by carrying out an interference lithography technique, and the first mask is made of a material which is substantially insensitive to the radiation used in the interference lithography technique, so that the second mask may be formed after the formation of the first mask without affecting the first mask.
The step of forming the second mask may then comprise the steps of: forming a photoresist layer on the face of the substrate supporting the first mask, and forming the pattern corresponding to the optical structure in the photoresist layer using the interference lithography technique.
According to a first embodiment of the present invention, the first and second masks are respectively made of metal and a photoresist material. The metal may typically consist of at least one of the following metals: nickel, chromium and gold.
According to this first embodiment, the step of forming the first mask comprises the steps of: forming a first layer on the substrate, the first layer being made of a material which is substantially insensitive to light; forming a photoresist layer on the first layer; patterning the photoresist layer using a UV exposure technique, so as to obtain a photoresist pattern corresponding to the first region of the substrate; etching the first layer using the photoresist pattern as a mask; and removing the photoresist pattern.
The above-mentioned step of etching the first layer may consist of a wet etching step.
According to a second embodiment of the present invention, the first and second masks are both made of a photoresist material, the first mask being however constituted by a photoresist material which has been heated in order to remove its sensitivity to light.
According to this second embodiment, the step of forming the first mask comprises the following steps: forming a photoresist layer on the substrate; patterning the photoresist layer using a UV exposure technique, so as to obtain a pattern corresponding to the first region of the substrate; and heating the photoresist pattern in order to remove its sensitivity to light.
According to a third embodiment of the present invention, the first mask is formed using a UV exposure technique, and the second mask is made of a material which is substantially insensitive to UV, so that the first mask may be formed after the formation of the second mask without affecting the second mask.
In this third embodiment, the step of forming the second mask comprises the steps of: forming a first layer on the substrate, the first layer being made of the material which is substantially insensitive to UV; forming a photoresist layer on the first layer; patterning the photoresist layer using an interference lithography technique, so as to obtain a photoresist pattern corresponding to the second region of the substrate; etching the first layer using the photoresist pattern as a mask; and removing the remaining photoresist pattern.
The step of forming the first mask then comprises the steps of: forming a photoresist layer on the second mask, and forming the pattern corresponding to the at least one optical device in the photoresist layer using the UV exposure technique.
The substrate used in the three embodiments above is preferably a silicon on insulator substrate.
The optical structure formed in the substrate typically consists of an array structure proximate to the optical device. In practice, the integrated optical circuit as obtained by the method according to the invention may comprise a waveguide, as the optical device, and an array structure having a frequency bandgap, as the optical structure. The array structure may take the form of a periodic array of holes or a periodic array of rods.